The present invention relates to a system for a working machine, wherein the system is adapted for springing the movement of a load during transport. The system comprises at least one hydraulic cylinder for operating the load, an accumulator, and a valve adapted to control a flow communication between the hydraulic cylinder and the accumulator. In particular, the invention relates to a working machine comprising the system.
The invention further relates to a method of springing an implement of a working machine during transport.
Below, the invention will be described in connection with the operation of a working machine in the form of a wheel loader. This is a preferred, but by no means limiting application of the invention. The invention can also be used for other types of working machines (or work vehicles), such as a backhoe loader, an excavator, or an agricultural machine such as a tractor.
When the implement of the wheel loader (for example a bucket or pallet forks) is brought into contact with a load and lifts it, the implement is preferably rigidly connected to the frame of the wheel loader. During transport of the load, however, particularly over an uneven ground surface, it is advantageous that the implement can move (sway) relative to the frame. In this way, the comfort of the operator is increased and material spillage from the implement during transport is reduced. For this reason, wheel loaders are equipped with load arm suspension. In a load arm suspension, the lift cylinders of the working machine are brought into flow communication with an accumulator. Thereby, the load arm becomes movable relative to the frame. The result is two movable masses instead of one.
In a loading cycle, usually an automatic activation and deactivation of the load arm suspension is used. For example, a gear dependent activation can be used, which means that the load arm suspension is active all the time except with the 1st front gear. The 1st gear is activated immediately before the bucket is pushed into the material (rocks, gravel, etc), and thereby the load arm suspension is deactivated. When the wheel loader reverses from the material after this, the reverse gear is activated and thereby the load arm suspension is activated once again.
WO 99/16981 discloses a load arm suspension system. The system comprises an accumulator which can be brought into flow communication with a piston side of the lift cylinders. The system further comprises a tank for hydraulic fluid which can be brought into flow communication with a piston rod side of the lift cylinders. The system comprises a plurality of valves for controlling the suspension. The system, and particularly the valves, is designed in such a way that pressure equalization between the piston side of the lift cylinders and the accumulator is performed automatically before the load arm suspension is activated. In this way, the previously occurring pitch movements of the implement when activating the load arm suspension are avoided to a large extent.
According to prior art, the damping characteristic is usually constant (that is to say non-variable) and thus has to be adjusted for operation with either an empty bucket, a full bucket, or to an intermediate position, which means that the damping characteristic does not become optimal for other load cases.
When lifting extremely heavy pallet loads according to prior art, there is a risk that a dumping movement of the assembly occurs, causing the pallet to hit the ground. The reason for this is that the accumulator does not have time for sufficient charging during the short phase before activation takes place.
Furthermore, when activating a load-arm suspension according to prior art, there is a risk of a thump in the machine when the valve controlling the flow communication between the hydraulic cylinder and the accumulator is opened.
Furthermore, when activating a load arm suspension according to prior art, there is a risk that the accumulator is charged to pressure which is too high, which results in energy losses when the accumulator is drained to tank. This problem is particularly pronounced during short-cycle loading, when the loading (and thereby the charging) is performed at a high frequency (two to three times per minute).
It is desirable to achieve a system which provides springing of the movement of the implement during transport and which creates prerequisites for a flexible and, from an energy point of view, efficient operation. More particularly, the invention seeks to create prerequisites for solving at least one of the above mentioned problems.
According to an aspect of the present invention, a system comprises a first control valve arranged on a conduit connecting to the piston side of the hydraulic cylinder, a second control valve arranged on a conduit connecting to the piston rod side of the hydraulic cylinder, a first pressure sensor for detecting a load pressure of the hydraulic cylinder, a second pressure sensor for detecting a charge pressure of the accumulator, and a control unit adapted to receive signals with information about the pressures detected by the pressure sensors and to generate control signals corresponding to the detected pressure for controlling the springing function.
In this way, the damping characteristic can be varied, for example, depending on the load case. During transport with an empty bucket, a softer, that is to say more undamped, movement is desired than during transport of a heavy load (when a relatively stiff suspension is desired). The damping characteristic can be varied, for example, by controlling the opening degree of the valve, which controls the flow communication between the hydraulic cylinder and the accumulator, based upon detected pressure levels.
Before activation takes place, pressure equalization has to be performed, in order not to get uncontrolled movements when activating the suspension. Thus, the accumulator should be brought to substantially the same pressure level as the one the lift cylinder (the piston side) has before it is connected. Thanks to the fact that the system comprises pressure sensors for detecting a load pressure of the hydraulic cylinder and a charge pressure of the accumulator, the suspension can be activated when the pressure in the accumulator is within a determined window (offset) compared to the lift cylinder. This accordingly means that a limited uncontrolled up or down movement is allowed.
Thanks to the fact that the system comprises pressure sensors for detecting a load pressure of the hydraulic cylinder and a charge pressure of the accumulator, the suspension can be activated according to determined, variable methods, depending on whether the pressure in the accumulator is lower or higher than the pressure in the lift cylinder.
When automatic activation and deactivation of the load arm suspension is concerned, the invention creates prerequisites for reducing the activation time. According to prior art, the load arm suspension can be deactivated when the pressure in the accumulator is low (empty bucket) and activated when the pressure in the lift cylinder is high. A large quantity of oil must then be filled into the accumulator and the time for activation can become long. One way of reducing this time is to sneak-fill oil into the accumulator to a certain specific pressure level when lifting takes place.
Furthermore, with this system a limit can be set on the maximum pressure in the accumulator. The control unit registers the pressure in the accumulator via its associated pressure sensors. When this pressure has reached a specific level, the control unit closes the valve which is connected to the accumulator. This can be used in order to increase the service life of the accumulator or, alternatively, to reduce the complexity, and thereby the cost, of the accumulator.
Furthermore, the arrangement with the first and second control valve offers large possibilities for controlling the activation of the suspension in an optimal way. In particular, it is not necessary to connect the pump and tank simultaneously to the lift function. The first and the second control valve are used together for lifting the load and lowering the load, respectively. Accordingly, the lift function is double-acting. The first control valve and the second control valve are actuatable independently of each other.
The hydraulic system is preferably load-sensing. This means that the pump detects the pressure (a LS signal) from the activated hydraulic cylinders. The pump then sets a pressure which is a certain number of bar higher than the pressures of the cylinders. This brings about an oil flow out to the control cylinders, the level of which depends upon how much the activated control valve is adjusted.
A second object of the invention is to achieve a corresponding method which provides springing of the movement of the implement during transport, and which creates prerequisites for a flexible and, from an energy point of view, efficient operation.
This is achieved by means of a method comprising the steps of detecting a charge pressure of the accumulator and a load pressure of the hydraulic cylinder, and controlling activation of the springing function based upon the detected pressures.
This object is also achieved by means of a method comprising the step of controlling a damping of the movement of the implement by variably controlling the opening degree of a valve adapted to control a flow communication between the hydraulic cylinder and an accumulator.
Further preferred embodiments of the invention and advantages associated therewith are apparent from the following description.